It is known as the Bergius process to obtain liquefied crude oil by the hydrocracking of a preheated paste consisting of finely divided coal, catalyst and heavy oil under an elevated temperature and pressure.
For carrying out the Bergius process practically, there have been employed so-called continuous tubular reactors. However, in any such tubular reactors, the residence time of paste within its respective tube is as long as approximately one hour, namely, the liquid/space velocity is about one and a complete mixing is carried out within the reactor by the agitation due to hydrogen injection.
It is obviously advantageous to carry out a reaction with a high liquid/space ratio as more paste can be treated in a reactor of the same volume. To achieve a high liquid/space ration, in other words, to shorten the residence time, it is necessary to conduct a reaction at an elevated temperature and in a short period of time.
Conventional reactors for direct liquefaction of coal were of the type of complete mixing vessels. It is theoretically known that an ideal tubular reactor can increase the liquid/space ratio of a first order reaction by 4 times and that of a second order reaction by 10 times compared with those in a complete mixing vessel when the reaction velocity constant is identical.
Since the direct liquefaction reaction of coal is a first order reaction, it is theoretically possible to reduce the residence time by one tenth to one twentieth with respect to that in a conventional reactor if the reaction temperature is somewhat raised.
The present inventors have achieved success in directly liquefying coal with a high liquid/space ratio by improving a tubular bubble tower type reactor.
It is known that, in fluid flow within a tube, there are considerable differences in the manner of mixing between fluid flow of a Reynolds number of 3.times.10.sup.3 or greater (turbulent flow range) and that of a smaller Reynolds number of 10.sup.1 -10.sup.3 regardless of whether the fluid is a liquid phase or gaseous phase.
In a fluid flow of a Reynolds number of 3.times.10.sup.3 or greater, the flow is a plug flow or quasi plug flow, and mixing in the flow direction is very little but the flow in the radial direction is completely mixed.
In considering the back mixing flow, the extent of back mixing (back mixing degree) is represented by the following equation; EQU back mixing degree=D/ud
wherein u: liquid flow velocity; d: tube diameter; and D: diffusion constant of back mixing. The back mixing degree ranges from 10.sup.1 to 10.sup.5 with a Reynolds number of 10.sup.2 to 0.2.times.10.sup.3 while it ranges from 0.2 to 0.7 with a Reynolds number of 10.sup.4 to 10.sup.5.